Hemodialysis involves the passing of the patient's blood on one side of a porous membrane and a physiological prescription electrolyte solution on the other side of the membrane. Through convection and diffusion the blood is equalized with the electrolyte solution and the patient's blood is cleansed. The membrane is usually part of the hemodialyzer.
The prescription electrolyte is the dialysate. The flow rate of the dialysate is typically 500 ml/min or greater. Thus, a typical 4 hour hemodialysis treatment would require over 30 gallons of dialysate. In a moderate size facility that is treating 25 patients per day, over 750 gallons of this dialysate fluid is required per day. This volume of fluid would present a handling and storage problem. To transport this much fluid every day would be very expensive and present massive logistical problems.
To help solve this problem, the dialysate used to treat patients has been delivered to the dialysis machine in concentrated form and diluted to useable dialysate by the dialysis machine. The standard concentration ratios that have been adapted by the industry for many years are 45X and 36.83X. Acid concentrates (Part A) along with the appropriate bicarbonate concentrate (Part B) is delivered to the hemodialysis machine and the hemodialysis machine further dilutes them with purified water to make the dialysate needed for the hemodialysis treatment. Instead of needing 30 gallons of liquid for each treatment, only three gallons (approx. 1 gallon of Acid concentrate and approx. two gallons of Bicarbonate concentrate) are needed per treatment. This has greatly reduced the cost of shipping, storage, and handling and reduced the logistical problems considerably. It is noted that most of the time the bicarbonate is delivered to the facility as powder and the acid is delivered as liquid.
Depending on the patient's needs, the concentrate is prescribed by the attending physician for that patient. In most dialysis facilities more than one formula is available to treat the patient population. The differences in the formulas may be as minor as a 2 mEq/l change in sodium or as major as a 200 mg/dl change in dextrose in the final dialysate. Most manufactures of acid concentrate offer over 50 different formulas to their customers. Custom formulated acid concentrates may also be offered.
Each hemodialysis patient typically needs to be treated three times per week. As the number of patients receiving hemodialysis has increased (it is estimated that currently over 230,000 patients in the U.S. receive hemodialysis treatments) there is still a large amount of acid concentrate that must be shipped and stored to treat these patients. Thus, concentrating the dialysate 36.83X or 45X has not resolved the problem entirely.
Several potential solutions to this continuing problem have been proposed over the years. First, the acid and bicarbonate concentrates could be made more concentrated. However, this is limited by the solubility of the electrolytes. There are physical limitations as to what volume of chemicals can be put into and maintained in solution. The change from 36.83X to 45X is an example of making the solution more concentrated. The 45X solution is close to this maximum solubility since the maximum amount of sodium chloride which can be placed into solution is about 3.3 lb. per gallon.
Another way of potentially solving this problem is to provide the Acid concentrate or Bicarbonate concentrate as a powder in a canister through which water is pumped. The output of the canister would be a saturated solution which is further diluted by the hemodialysis machine. See, e.g., Jonsson et al. U.S. Pat. No. 4,784,495, which discloses a method using the canister for the bicarbonate concentrate Part "B". Jonsson et al also shows a method to use sodium chloride in a canister. However, it is evident that not all of the electrolytes needed for hemodialysis will dissolve at a sufficient rate to allow the canister process to be used exclusively. Thus, it is necessary to also provide a liquid solution to the machine. Moreover, due to the variation in dissolution rates only one chemical can be used in each canister, which complicates the design of the dialysis machine. This process also needs a special machine to dilute and mix the concentrates.
Another potential solution is to deliver the chemicals to make an acid concentrate in a powder form to the dialysis facility. At the hemodialysis facility, the powder would be reconstituted into acid concentrate by adding water and then supplied to the hemodialysis machine, where it is further diluted with water and Part "B" concentrate to make the dialysate. The powder may be delivered to the facility in large pre-measured quantities which are made up at one time. See, e.g., Harm et al., U.S. Pat. No. 4,734,198. This solution does reduce the shipping cost to an optimal level, but it creates several new problems. Powders in large quantities are heavy and difficult to handle. Moreover, sometimes the powders are slow to dissolve. In addition, the acid required in the formulas is not readily available in powder form and is expensive. Some of the other chemicals used in these powder formulas are also more expensive in powder form. For example, calcium chloride in powder form typically costs $1.88/lb. However, in liquid form calcium chloride can typically cost as low as $0.10/lb.
Lastly, using powder creates multiple mixing steps. When these powders are mixed into solution they are normally added to a starting volume of water and then the solution is topped off with additional water to a fixed final volume. This three step process of filling the tank, adding and mixing the chemicals and topping off requires extra steps which are not desirable for the dialysis facility.